Due to rapid evolution and use of electronic technologies problems associated with incidental electromagnetic radiation have increased significantly. The trend to use of increasingly higher equipment operating frequencies leads to increasingly greater amounts of electromagnetic noise and interference emissions from the equipment. At the same time, the electronic components of the equipment have become smaller and more sensitive, making them more susceptible to adverse influence or damage by electromagnetic radiation to which they are exposed. Consequently, more strigent performance standards and regulations have been implemented that require new equipment to be designed and constructed with higher shielding characteristics that provide for control of emissions and protection against interference from incidental electromagnetic radiation.
Older electronic equipment, on the other hand, while constructed in compliance with standards effective at the time of their manufacture, often lack sufficient shielding to protect them from current levels of electromagnetic interference (EMI) to which they are exposed and, in many cases, are themselves a source of EMI. When located in clusters or confined to small outdoor areas such as building rooftops, equipment compounds, boat or ship superstructures, etc., the problems associated with inadequate EMI shielding become more acute as proximity makes it more likely that each piece of equipment will adversely affect, and be adversely affected by, its neighboring equipment. Such equipment includes exterior-mounted components of instrument systems, radio- and telecommunication systems, marine navigation systems, aircraft detection and control systems, electronic warfare systems, and the like, which, except for the lack of adequate EMI shielding, have nothing wrong with them and are expected to be usable for many years. It has become very desirable to upgrade these systems by providing them with sufficient EMI shielding to ensure their continued operation. It is also very desirable that modifications to equipment for EMI shielding be added quickly, inexpensively, and on-site so that the much larger systems, for example, weather stations, ships, airports, military facilities, etc., of which they are key elements are not rendered ineffective or inoperative for extended periods of time.
Exterior-mounted equipment needing EMI shielding are often mounted and housed in reinforced plastic enclosures in such a way that EMI shielding modifications to the inside surfaces of the enclosure or to the equipment within the enclosure are impractical or ineffective. In these instances, EMI shielding modifications must be made outside the enclosures, thus exposing them to a variety of weather and outdoor environments. Weather resistance and corrosion resistance are particularly important considerations for applications involving marine or coastal environments.
One method for providing EMI shielding to exterior-mounted equipment enclosures is to paint them with an electrically-conductive paint, for example, with paints filled with Nickel or Zinc particles. Such paints are effective in providing EMI shielding when freshly applied, however, their weather resistance and corrosion resistance in marine environments are such that they must be reapplied at frequent intervals, typically 3 to 6 months. Their performance and use lifetime is somewhat better in less severe environments.